Electron tubes



July 11, 1961 E. A. FROMMHOLD ELECTRON TUBES 2 Sheets-Sheet 1 Filed Sept. 4, 1959 July 11, 1961 E. A. FROMMHOLD ELECTRON TUBES Filed Sept. 4, 1959 Twill.

2 Sheets-Sheet 2 United States Patent 2,992,350 ELECTRON TUBES Ernst Alfred Frommhold, Dresden, Germany, assignor to VEB Vakutronik, 'Dresden, Germany Filed Sept. 4, 1959, Ser. No. 838,323 1 Claim. (Cl. 313-407) This invention relates to electron tubes for application in circuits requiring a high input impedance and in par ticular to the special vacuum tubes known as electrometer tubes and their associated circuits.

While circuits having high input impedances are extremely useful in many applications a high input impedance is most essential for measuring small currents.

Many methods for achieving a high input impedance in electron tubes are known. The main detriment thereto, grid current, may be easily reduced to negligible values by a suitable negative bias. An increase of negative grid bias of approximately .9 volt at a cathode temperature of ll60 K. in a tube may reduce the grid current by a factor of Negative grid bias beyond the point at which positive grid current is initiated causes a reversal and an electron flow away from the grid.

Residual grid current is maintained at a low level by resorting to one of the following or a combination of the following remedies in the production and use of ordinary electrometer or other tubes. A cathode with a low operating temperature may be used; the positive driving voltage of electrodes attracting electrons may be reduced by means of small batteries to a value below the ionization and exciting potential of the residual gases; an especially high vacuum may be used; the insulating properties of the support for the control grid may be improved by use of quartz members; the insulating parts in the ambient atmosphere and connected to the control electrode may be coated with an insulating non-hygroscopic layer; theinsulators found within the tube may be protected and, insofar as possible, the glass envelope, from contamination or fogging with conductive substances; the control electrode and the space between the control electrode and the other electrodes may be dimensioned as small as possible to reduce the ion buildup in this space from cosmic and radioactive ambient rays; the control grid may be equipped with a coating of high work function; the control grid may be constructed to rapidly dissipate heat; the tube and circuit may be shielded from extraneous rays; the entire circuit and tube is incorporated in a suitable isolating dehutnidified structure by means of a drying agent.

The optimum characteristics for a control grid in electrometer vacuum tubes may be achieved by the insertion of a space charge grid between the cathode and the control grid. By insertion of a space charge grid the positive ions from the control grid are restrained and because of the development of a virtual cathode the amplification factor is improved. Despite these measures a residual current is always present.

Electron tubes in which the electrodes are arranged in order of cathode, anode, control grid have been used. This arrangement, however, which is known as an inverted tube until now did not produce any advantage over the specially made grid controlled tubes subjected to the measure for improvement identified above. For this reason, inverted tubes have not been used other than in exceptional cases.

It became apparent that the residual current remaining in a grid controlled electron tube is principally traceable to secondary emission especially low energy roentgen rays (Bremsstrahlung). Therefore, in accordance with the invention an inverted tube is subjected to the above stated measures for reduction of the residual current and is used as an electrometer tube.

In grid controlled electron tubes with space charge grids, the greatest part of the cathode current is not susceptible to control, and, therefore, not susceptible to amplification but still causes secondary emission.

It is an object of this invention to provide electron tubes which achieve more complete cathode current control and also obtain the advantages of grid controlled tubes having a space charge grid inserted therein.

It is a further object of the invention to provide an electron tube having an input impedance higher than'was formerly possible. i

The tube in accordance with the invention and as compared to a normal grid controlled tube, provides the advantage of minimum residual current under identical transconductances or the advantage of greater transconductance at the same residual current.

For a more complete understanding of the nature and scope of the invention, reference may be had to the following detailed description which may be read in connection with the accompanying drawings in which:

FlG. 1 is a plan sectional view of the device in accordance with the invention and showing the arrangement of electrodes in the tube used therein;

FIG. 2 is a schematic representation of the dynamic and current characteristics of the tube constructed in accordance with the invention; and

FIGS. 3 and 4 are schematic circuit diagrams which centrally located heated cathode 14, a grid shaped anode. 16 and a massive control electrode 18 surrounding the anode.

The cathode 14 is constructed of a material of low operating temperature such as an oxide. The envelope eucloses the tube under conditions of an extremely high vacuum and the control electrode 18 is supported withinthe envelope 12 by means of quartz members 19.

A number of contacts 20, 22, and 24 are respectively connected to the cathode, the anode and the control electrode by means of electrical wires 26 passing through the envelope. A suitable circuit 25 such as the one shown in FIG. 3 is connected to the contacts 20, 22 and 24. The entire tube is assembled under conditions of controlled humidity and temperature for maximum purity of components. A coating 27 of high work function surrounds the control electrode 18 which is adequately spaced from the envelope 12 for rapid dissipation of heat. A coated shield 30 encapsulating the circuit 25 and the tube 10 is provided with suitable openings for power connections, and a probe connected to the tube 10 which protects the contents from extraneous radiation and humidity by means of a drying agent 32, such as calcium carbide.

The insulating parts within the shield 30 are provided with a non-hygroscopic insulating coating.

Referring to FIG. 2 the abscissa is graduated in grid or control electrode volts, the upper ordinate in microam-peres of anode current and curve 34 is representative of the dynamic characteristics of an ordinary electrometer vacuum tube. Curve 36 illustrates the dynamic characteristics of a vacuum tube constructed in accordance with this invention. It will be seen that the rate of rise which is indicative of transconductance of the tube is far greater in the region of lower bias for the tube constructed in accordance with this invention. The average transconductance indicated by curve 34 is 15 micromhos whereas on the steep side of the characteristic of curve 36 the transconductance is seen to be 60 micromhos. The lower ordinate is graduated in microamperes of grid or control electrode current and curve 38 illustrates the control grid current of a familiar subnriniature tube with a space charge grid. It should be noted that in the ordinary tube the current rises and remains substantially high and constant over a large portion of the range of the control grid. Curve J40 depicts the variable characteristic of the control electrode current in the tube constructed as in FIG. 1 and shows it to be substantially lower, particularly at the point of intersection 41. with the abscissa at which point the input impedance of the tube is practically infinity.

A typical current measuring circuit 25 constructed for use of a tube in accordance with the invention is shown in FIG. 3 with low current input 42 to a tube 44 biased by means of an electric cell 46, having a directly heated filamentary cathode 48, an anode 49 and a massive control electrode 50 connected to the input. An electric cell 52 for the cathode 48' is connected to the negative portion or a 6 volt B battery 54. A sensitive galvanometer 56 is connected to the anode 49. .A compensating voltage 8 is connected in series with a variable compensating resistance 60 connected to the anode 49. An output for further amplification may be taken across the meter at terminals 62 and 64.

The entire circuit 25 with the exception of an input probe may be advantageously encapsulated in the shield 30 described in- FIG. 1 with the appropriate parts of the meter on display or positioned outside the shield on a suitable panel.

When used as described above the tube and the circuit provides an especially high input impedance for such purposes as measuring extremely small currents.

The tube disclosed herein may advantageously be employed in an ordinary measuning bridge circuit with a meter appropriately connected between anodes or two inverted tubes of the type described in FIGS. 1 and 3.

In FIG. 4 the tube 10 disclosed herein is advantageously employed in a measuring bridge generally designated 68. The latter includes a measuring electrometer tube 70, a tube 72 for the compensating current, a heater voltage source in the preferred form oi a battery 74-, a pair of variable resistances 76 and 78 for suitably varying the biases for the control electrodes of tubes and 72., respectively, an accompanying biasing voltage source 80, a pair of plate resistors 81 and 82, a balancing potentiometer 84, a suitable source of positive anode voltage in the preferred form of a battery 85', a galvanometer 86 and a pair of amplifier terminals 88 and 90, all connected as shown. The input voltage is connected to terminals 92 and 94.

The inputs of the tubes connected as illustrated in FIGS. 3 and 4 provide impedances which may be four times the input impedance of a normal electromcter tube.

While certain changes may be made in the above described construction and different embodiments of the invention may be made without departing from the spirit and scope thereof, it is intended that all matter contained in the above descniption and shown in the accompanying drawing shall be illustrative and not in a limiting sense.

What is claimed is:

In an electronic arrangement comprising a vacuum tube including an envelope, a cathode within said envelope, a control grid said envelope spaced from said cathode, an anode said envelope spaced intermediate said cathode and said control electrode, means in said envelope for supporting said cathode and said anode; and means for reducing the Bremsstrahlung radiation within said envelope during operation of said tube, said means including an encapsulating chamber, drying means in said chamber for dehumidifying said chamber, a radiation shield within said chamber, a plurality of quartz members positioned within said envelope for sup porting said control electrode, and circuit means in said chamber projecting into said envelope for electrical connection to said anode, said cathode and said control electrode and means for energizing said cathode, said anode and said control electrode.

References Cited in the file of this patent Henney: Electron Tubes in Industry, McGraw-Hill Book (30., Inc, New York, 1934, pp. 90-91.

Prescott: The Use of Multigrid Tubes as Electrometers, Rev. of Science Insts, August 1949, pp. 553-55 7. 

